This invention relates generally to gas turbine engines and more particularly, to gas turbine engines working on an inverted Brayton cycle.
Gas turbine engines typically include a compressor for compressing a working fluid, such as air. The compressed air is injected into a combustor which heats the fluid causing it to expand, and the expanded fluid is forced through a turbine or turbines. The compressor may include a low pressure compressor and a high pressure compressor.
In some engine applications, such as on oil platforms, a heavy penalty is paid for fuel burned because of emissions taxes. Therefore, in such applications, it is desirable to provide increased power output at a same fuel flow (i.e., decreased specific fuel consumption) as is currently used. For such applications, the engine also preferably is low in weight and has a small volume compared to other power plant installations.
One known cycle which provides increased power output as compared to a direct open cycle is commonly referred to as an inverted Brayton cycle. In an inverted Brayton cycle, the high temperature exhaust gases from a main engine are expanded from high temperature and approximately atmospheric pressure to sub-atmospheric pressure. The air is then cooled at approximately constant, low pressure, and is recompressed to atmospheric pressure and discharged.
With an inverted Brayton cycle and due to the low pressures, the engine components, and particularly the turbine and compressor, must be much larger than those of a gas turbine working on a direct open cycle with high-pressure combustion. Particularly, additional power can be generated with an inverted Brayton cycle. Further, although an inverted Brayton cycle generates higher powers with any given compressor-turbine combination, its thermal efficiency is lower than in a high temperature direct open cycle. Specifically, the inverted cycle requires a cooler, which is not necessary in the direct cycle. The cooler adds a pressure drop in the cycle and takes heat away from the cycle, which adversely affects its thermal efficiency.
It would be desirable to provide a gas turbine engine which provides increased power output at a same fuel flow as is currently used in some known applications. It also would be desirable to provide such an engine which is low in weight and has a small volume compared to other power plant installations.
These and other objects may be attained by a gas turbine engine working on an inverted Brayton cycle (IBC) which provides increased power output at a same fuel flow as is currently used in some known applications. In addition, a known gas turbine engine can be easily modified to implement the IBC, and such engine is relatively low in weight and has a small volume as compared to other power plant installations. The advantages of increased power and reduced specific fuel consumption therefore are believed to be achieved with such an engine.
More specifically, and in one embodiment, a parent or main engine includes a compressor coupled by a first shaft to a high pressure turbine. A combustor is located in the flow intermediate the compressor and high pressure turbine. A free wheeling power turbine is located downstream of the high pressure turbine, and the power turbine is coupled to a load by a second shaft.
The flow from the power turbine is supplied, e.g., via ducts, to an inverted Brayton cycle (IBC) axial turbine coupled to an axial compressor by a third shaft. A heat exchanger is located in the flow intermediate the axial turbine and axial compressor. Cooler air flow from the heat exchanger is supplied, e.g., via ducts, to the high pressure axial compressor or booster.
In operation, the working fluid (e.g., air) is compressed by the compressor, and the compressed air is injected into the combustor which heats the air causing it to expand. The expanded air is forced through the high pressure turbine and the expanded air is supplied to the power turbine. Energy from the power turbine is transferred to the load via the second shaft.
At least a portion of the air flow from the power turbine is supplied to the IBC axial turbine which operates as an expander. In one specific embodiment, the expanded air flow is supplied to an inlet of the heat exchanger at or near 4 psia pressure, where at least a portion of the air flow is cooled from about 600 degrees Fahrenheit to 89 degrees Fahrenheit. Some of the cooled gas flow is supplied to the booster. Air from the booster is discharged into the atmosphere.